Fuel cells are used as an electrical power source in many applications. In particular, fuel cells are proposed for use in automobiles to replace internal combustion engines. A commonly used fuel cell design uses a solid polymer electrolyte (“SPE”) membrane or proton exchange membrane (“PEM”) to provide ion transport between the anode and cathode.
In proton exchange membrane type fuel cells, hydrogen is supplied to the anode as fuel, and oxygen is supplied to the cathode as the oxidant. The oxygen can either be in pure form (O2) or air (a mixture of O2 and N2). PEM fuel cells typically have a membrane electrode assembly (“MEA”) in which a solid polymer membrane has an anode catalyst on one face, and a cathode catalyst on the opposite face. The anode and cathode layers of a typical PEM fuel cell are formed of porous conductive materials, such as woven graphite, graphitized sheets, or carbon paper to enable the fuel and oxidant to disperse over the surface of the membrane facing the fuel- and oxidant-supply electrodes, respectively. Each electrode has finely divided catalyst particles (for example, platinum particles) supported on carbon particles to promote oxidation of hydrogen at the anode and reduction of oxygen at the cathode. Protons flow from the anode through the ionically conductive polymer membrane to the cathode where they combine with oxygen to form water which is discharged from the cell. The MEA is sandwiched between a pair of porous gas diffusion layers (“GDL”) which, in turn, are sandwiched between a pair of non-porous, electrically conductive elements or plates. The plates function as current collectors for the anode and the cathode, and contain appropriate channels and openings formed therein for distributing the fuel cell's gaseous reactants over the surface of respective anode and cathode catalysts. In order to produce electricity efficiently, the polymer electrolyte membrane of a PEM fuel cell must be thin, chemically stable, proton transmissive, non-electrically conductive and gas impermeable. In typical applications, fuel cells are provided in arrays of many individual fuel cells arranged in stacks in order to provide high levels of electrical power. Although the catalyst layers used in fuel cells work reasonably well, there is a need for improved fuel cell catalyst layers.
Carbon black and carbon nanotubes are preferred support for fuel cell electrocatalysts due to their excellent electronic conductivity, good mechanical and chemical stabilities, and low cost. Catalyst layers typically use platinum and/or a platinum alloy which are made into very fine nanoparticles in order to improve overall activity by enhancing the surface area. However, the inert surface of the carbon makes it very difficult for metal to attach. Nucleation of platinum on carbon surface has proven to be challenging resulting in large platinum particle size and particle migration and agglomeration. Moreover, the high surface energy of platinum makes it very difficult to coat a thin and smooth surface. Scanning electron micrographs illustrate the tendency of these prior art coatings to agglomerate without forming a smooth homogeneous layer. The poor interaction between platinum and carbon surfaces makes it difficult to nucleate and form a uniform film. In particular, due to poor nucleation and layer growth, a relatively high minimum-layer thickness is required in order to ensure electric conduction.
A commercially available nanostructured thin film (NSTF) catalyst is made by sputtering Pt on a self-assembled perylene red dye support. The support is electronically non-conductive, so electron must transport through the coated continuous layer of Pt. On the other hand, this catalyst type is more durable than conventional carbon-supported Pt nanoparticles due to the smooth Pt surface and superior stability of the support. Some of the prior art sputtered films are observed to be non-uniform due to the line of sight nature of the sputtering process.
Accordingly, there is a need for improved materials for forming fuel cell catalyst materials.